1. Field of the Invention
This invention relates to tracking control arrangements and, more particularly, to such arrangements having particular application in reproducing digital signals recorded in substantially parallel tracks with improved tracking control over the heads used to reproduce those digital signals. The invention is especially useful in digital audio tape (DAT) and in video signal reproduction.
2. Description of the Prior Art
Tracking control techniques have long been known to position one or more transducers, namely, playback heads, properly over data tracks from which previously recorded signals are reproduced. Such tracking control techniques are used in servo control circuitry for digital mass storage devices, such as magnetic disk drives, video record/reproduce devices (referred to typically as video tape recorders) and in recently introduced digital audio record/reproduce systems, such as digital audio tape (DAT) recorders. In VTR and DAT devices, video or audio information is recorded in parallel slant tracks across a magnetic tape by a helical-scan rotary device. Preferably, digital codes are used to represent the video and audio information, such as pulse code modulated (PCM) signals. It is appreciated that PCM recording provides low loss, high quality signal reproduction.
In a typical tracking control arrangement used with rotary heads (the principal of which also finds ready application in fixed head devices, such as mass storage devices), a control signal is recorded when the useful information (i.e. the digital signals representing video or audio information) is recorded; and this control signal, when played back, is used to determine any deviation of the position of the playback head from the center of the track being scanned thereby. For example, in one prior art embodiment, the control signal is recorded by a stationary control head along a longitudinal edge of the magnetic tape on which the slant tracks of digital signals are recorded. The rotary phase of the scanning heads is compared to the phase of the reproduced control signal; and any phase differential therebetween is used to adjust the position of the heads relative to the scanned tracks.
As VTR and DAT systems became miniaturized, constraints on size and volume increased, making it desirable to eliminate various mechanical elements. The stationary control head, whose primary function was to reproduce pre-recorded control signals, thus became surplusage. Accordingly, it has been proposed in Japanese Laid Open Patent Application No. 59-112406 to provide a tracking control technique which does not rely on the aforementioned stationary control head. In that proposal, the rotary transducer arrangement is used to sense tracking errors.
The inherent ability of a PCM signal to be processed, such as to be timebase compressed, time division multiplexed, etc., allows various control signals to be recorded in the same track as the PCM signal but without interfering therewith. Such non-interfering combination of different types of signals is not readily available or easily attained when the information is recorded in analog form. Notably, a PCM signal need not be recorded or reproduced continuously but, rather, samples may be recorded in spaced apart locations, thereby facilitating the interspersing of other controlling information therewith. Thus, tracking control for VTR and DAT systems can be obtained by recording tracking information signals in the same track, but at different discrete areas, as the PCM signals.
In one prior art technique in which tracking information signals are recorded in the same tracks as PCM signals, plural rotary heads are angularly spaced on a drum, with each head scanning a single track such that one track at a time is recorded or reproduced. PCM signals are recorded in a relatively long data area which occupies the center portion of a track, and the tracking information signals are recorded in leading and lagging positions relative to the data area. In a simplified form, the tracking information signal comprises a pilot signal and, in addition, a synchronizing signal also is recorded in each track and spaced from a respective pilot signal. Thus, in one track a synchronizing signal may be recorded, followed by a pilot signal, followed by the data area, followed by another pilot signal which, in turn, is followed by yet another synchronizing signal. In an adjacent track, a pilot signal may be recorded, followed by a synchronizing signal, followed by the data area, followed by another synchronizing signal which, in turn, is followed by yet another pilot signal. During a recording operation, each head used to record PCM, pilot and synchronizing signals exhibits a width which is wider than the resultant track. Thus, a successive track slightly overlaps a preceding track such that the tracks are adjacent one another without the provision of guard bands. The recording of a later track tends to "over record" a narrow longitudinal portion of a preceding track.
As is known when recording tracks of high density, adjacent tracks should be recorded with heads having different azimuths. By reason of the phenomenon known as azimuth loss, a signal of relatively higher frequency recorded with one azimuth will not be picked up to any significant degree as cross talk when an adjacent track is scanned by a head of different azimuth. However, signals of relatively lower frequency do not exhibit azimuth loss and, thus, will be picked up as cross talk. Thus, when a head of, for example, azimuth A scans a track recorded previously by a head of the same azimuth A, any low frequency pilot signal in an adjacent track recorded with azimuth B will, nevertheless, be picked up as cross talk. The amplitude of this cross talk pilot signal component is a function of the tracking error of the head relative to the track which it is scanning. Based upon this tracking error, remedial efforts are made to return the head to its proper tracking position, such as aligned with the center of the track, as by increasing or decreasing the tape speed which results in a realignment of the head relative to the track.
Whereas the pilot signal is recorded with a relatively low frequency to minimize azimuth loss and thus permit cross talk to be detected, a synchronizing signal is recorded at a relatively high frequency such that when the synchronizing signal in an adjacent track is picked up, azimuth loss attenuates its amplitude. Of course, the synchronizing signal recorded in a given track is reproduced by the head which scans that track. By judiciously recording the pilot and synchronizing signals at discrete times in each track, the resultant track pattern has a synchronizing signal in one track adjacent the pilot signal in an adjacent track. Thus, when a given track is scanned, at the time that the synchronizing signal in that track is reproduced, tracking control circuitry may be triggered to detect any cross talk pilot signal that is picked up from an adjacent track. For example, when the head of azimuth A senses the synchronizing signal which had been recorded with azimuth A previously, the tracking control circuitry is triggered to sense any pilot signal which had been recorded in an adjacent track with azimuth B and which now may be picked up as cross talk by this head. Of course, any synchronizing signal which had been recorded in the adjacent track with azimuth B will not be picked up as cross talk by the scanning head of azimuth A by reason of the aforementioned azimuth loss phenomenon.
However, the aforementioned technique of recording relatively low frequency pilot signals and higher frequency synchronizing signals in discrete locations in each track such that the synchronizing signal in one track is positioned opposite the pilot signal in an adjacent track results in a relatively complicated recording scheme. The recording of signals of substantially different frequencies to take advantage of the azimuth loss phenomenon adds to this complexity.